Two stroke internal combustion engines have one inlet-compression and one power-exhaust stroke with every complete revolution of the crankshaft. The inlet, compression, power and exhaust strokes are completed in just two strokes of the piston that result from a single 360 degree revolution of the crankshaft. These engines are usually high speed, low weight/power machines which are commonly used to power motorcycles, go-carts, snowmobiles, jet-skis, small aircraft, etc. Two stroke cycle internal combustion engines as used in motorcycles and in the other devices described above normally use a reed type of inlet valve to admit the fuel-air mixture into the crankcase. The reed type inlet valve also acts as a check valve to prevent escape of the fuel-air mixture from the inlet port when the mixture is compressed in the crankcase by the downward motion of the piston until bypass ports that connect the crankcase to the cylinder are opened by the piston. The piston also uncovers exhaust ports in the cylinder wall (usually just prior to the inlet valve opening event). The expanding exhaust gases leave the cyllnder through the exhaust ports and the expansion wave created by their flow helps the compressed inlet gas charge to enter and fill the cylinder before the piston covers and seals the exhaust and inlet ports on its upward stroke in the cylinder where it compresses the fresh charge. A spark or glow plug fires the charge near the top of the stroke and combustion increases the internal gas pressure to the point where it performs useful work as it expands when the piston descends in the cylinder to repeat its two stroke cycle. The upward stroke of the piston increases the internal volume of the crankcase, thereby lowering the internal pressure in the crankcase. The pressure differential between the ambient atmosphere at the carburetor inlet and the crankcase interior causes air to flow through the carburetor venturi, through the carburetor to the crankcase induction pipe and through the reed valves into the crankcase. Because the power output of these engines is dependent on the weight of the air-fuel charge that is induced to flow into the engine cylinder via the crankcase during each stroke, it is imperative to minimize the flow discontinuities in the inlet system that cause pressure drops and accompanying reduction of air-fuel mixture flow into the engine.
Because the only energy that causes air to flow into the engine is the pressure differential between the crankcase and atmosphere acting on the area of flow, any obstruction or turbulence created pressure drop in the induction system will reduce the flow into the engine, hence reducing the available power from a given engine configuration. Manufacturers and motorcycle enthusiasts, especially those engaged in motorcycle racing, hill climbing and other contests, as well as racers operating go-carts, snowmobiles, jet-skis, small aircraft, etc. go to great lengths to maximize the airflow into the engines that power these machines. There are many devices on the market that claim to improve power available from factory standard motorcycle engines. Many of these claim to improve flow into the engine by reducing pressure drop in the induction system, by tuning the reed valves etc.
The inventors have found that the power enhancing attachments that were available did not attack the fundamental problem of smoothing and guiding the air-fuel flow into the immediate vicinity of the reed valves. The inventors have also researched the literature and discuss the following patents which may be construed as having similar function:
U.S. Pat. No. 4,228,770 by Boyesen describes an inlet flow smoothing device which comprises an airfoil shaped obstruction that is used to reduce the inlet flow cross sectional area upstream of the reed valve inlet frame. This obstruction will increase the flow velocity as claimed, because neglecting losses, in incompressible subsonic flow, for a given total pressure, the flow in a duct system can be defined by the following relationships given in equation form: EQU (.rho.)(A)(V)=Q (constant)=mass flow rate, lbs(mass)/sec PA0 where: PA0 D=drag force, lbs. PA0 C.sub.d =drag coefficient=C.sub.dw +C.sub.df +C.sub.db where: PA0 d.sub.b =diameter of the base PA0 d=diameter of the body.
.rho.=density, mass/unit volume PA1 A=cross sectional area, ft.sup.2 PA1 V=flow velocity, ft/sec. PA1 C.sub.dw =wave or form drag coefficient PA1 C.sub.df =skin friction drag coefficient PA1 C.sub.db =base drag coefficient PA1 V=flow velocity, ft/sec PA1 S=reference area, ft.sup.2.
Hence, any restriction in cross sectional area, A, that is caused by an obstruction submerged in the flow, for a constant flow rate, Q, results in increased local velocity at the obstruction.
The point of this discussion is that any object, streamlined or not, which is submerged in a flowing gas stream experiences an aerodynamic drag which is comprised of three components: (1) wave or form drag, which is the resistance of the object to airflow caused by its shape, (2) skin friction drag caused by the shearing action of the gas flowing over the surface and (3) base drag caused by the low pressure separated region behind the immersed body. The forces that are aerodynamically generated on the body take energy from the flowing air and result in a reduced available pressure flowing into the engine. The drag of many types of airfoils have been calculated and experimentally determined. Many tables are available from textbooks and from NACA reports.
The equation for aerodynamic drag of a body submerged in a flowing airstream is given below: EQU D=C.sub.d .rho.V.sup.2 S/2
where:
Hoerner, "Fluid Dynamic Drag" 1965 Edition, page 6-1, shows that for streamline two dimensional shapes, the value of C.sub.dw is in the order of 0.1, friction drag, C.sub.df is approximately 0.05 (page 6-9), and base drag=C.sub.db. C.sub.db =(0.029)(d.sub.b /d).sup.3 /.sqroot.(C.sub.dw), (Hoerner, pages 3-20) where:
If one calculates or measures the drag caused by air flowing past a submerged body, it is apparent that a considerable portion of the dynamic pressure available to pump air into the engine is lost by flow resistance around the submerged body. Although it is likely that such an inserted vane device (described by Boyesen) will actually boost available power in an engine whose induction system is designed with induction passages that have unnecessary area increases in the vicinity of the reed valve inlet, it became apparent to the inventors that a better method of improving flow to the inlet valves would not place an obstruction in the flow with its inevitable losses. The inventors made an "annular" nozzle that changes the shape of the inlet passage to smooth the flow into a duct. It has a gradual taper on the exit and acts like a venturi. The only flow losses caused by the insertion of the present invention into an engine inlet are increased skin friction losses caused only if the air has to flow over increased surface area and separation or wake losses only if the invention causes separation. These losses are negligible for the invention which is a special form of a venturi. It is well known to engineers with ordinary knowledge in fluid flow that the velocity drop of fluid flowing through a venturi is of the order of one percent (Mechanical Engineers' Handbook, Fourth Edition, Lionel S. Marks, page 253). No device which is submerged in the flow, such as an airfoil or streamlined shape has such a minor flow loss. In Engineering Aerodynamics, revised edition, sixth printing, April 1943, Diehl states on page 192 that "In addition to the general downwash field behind a lifting wing, there is a narrow wake of highly turbulent flow and fairly low resultant velocity that persists many chord lengths down-stream. The effective velocity is reduced more than 10% for a thickness about equal to the wing depth . . ." This results in a 19% loss in dynamic pressure behind the wing.
Boyesen, in his later invention, U.S. Pat. No. 4,474,145, describes a three dimensional insert that protrudes into the reed valve cage to accelerate the flow and in some cases to cause a vortex to form in the airstream. Unlike the invention described herein by these inventors, Boyesen's device protrudes three dimensionally into the airstream and reduces the total pressure of the air flowing into the engine because it takes energy from the airstream in the form of drag and separated wake turbulence. Although Boyesen's inventions could benefit specific engine designs by helping air to flow through the valves, the invention that is the subject of this application performs the same function without the inherent losses caused by a plug or vane that is submerged in the flowing airstream.
Because no satisfactory substantially loss free means of smoothing airflow into the inlet valves of reed valve equipped two-cycle engines were available on the market for use with conventional factory standard motorcycle engines, the inventors herein have built and tested annular flow smoothers for insertion into the inlet ducts of factory standard motorcycle engines. The invention comprises an annular substantially rectangular hollow insert that replaces the factory standard inlet duct insert. In some cases, there is no factory standard insert and the invention sizes his insert to fit into the opening of the roof prism shaped reed valve inlet cage. The insert fits into a recess in the roof prism shaped reed valve inlet cage and has extending from the insert (in the direction of flow) tapered contoured extensions that extend along opposite parallel triangular interior walls of the inlet cage to smooth the flow and cause it to expand outward slowly as does the exit section of a venturi. The action of the tapered extensions in causing the air to expand smoothly outward towards the two parallel sides of the reed valve cage without boundary layer separation, increases air flow through the engine inlet valves and increases the power output of the engine which depends directly on the weight of the inlet air that enters the engine during each revolution of the crankshaft.